Oil exploration involves evaluating reservoirs to determine the movement or absence of oil, gas, or water as the reservoir fluids are produced. Understanding movement of gas in reservoirs is important to the prevention of premature breakthroughs and optimization of reservoir performance. It is known to use gravity borehole tools to measure characteristics of geologic formation, particularly in the exploitation of hydrocarbon reservoirs found in geologic formations or in the subsurface storage of carbon dioxide or water.
The process of measuring physical properties of earth formations beneath the surface of the earth is commonly referred to as “well logging”. It comprises the step of lowering sensors or testing equipment mounted on robust tool bodies into a wellbore drilled through the earth. When the tool is suspended from an armored cable the process is more specifically referred to as “wireline” well logging. Alternative conveyance techniques as known in the art include lowering the instruments mounted on drill pipe, casing or production tubing or on coiled tubing. The drill pipe conveyance technique, in particular, is known as “logging while drilling” when measurements are performed during the actual drilling of a wellbore.
Borehole gravity measurements are a direct measure of the bulk density of the formation surrounding a wellbore. Typically gravity data are taken at different vertical depths or stations along the wellbore. The basic principle of borehole gravity measurements is that the change in gravity relates directly to the bulk density contrast of the formation, the distance from the stations and the density contrast body. The bulk density in turn is directly related to grain densities and the pore fluid (gas, oil or water) densities and porosity of the formation. Several gravity measurement tools are commercially available. U.S. Pat. No. 5,970,787 to Wignall describes a tool for conducting gravimetric survey downhole in an earth formation.
One limitation to using gravity sensors is that the accuracy of the gravity measurement may be insufficient for making gravity density measurements in boreholes. Gravity sensors are extremely sensitive to vibrations and these vibrations may throw the gravity sensors out of calibration. Further, even if the sensor remains calibrated after being subjected to vibrations the sensor will take time to settle which is undesirable as it reduces the logging speed and increases the chance of having the tool stuck in the wellbore. Minimizing both noise and vibrations in the gravity measurements may increase this accuracy.